Support using VP9 video codec in AppRTCDemo.

- Add peer connection Java API to initialize field trial string.
- Add setting option to select VP8 or Vp9 as default video codec.
- Minor code clean up and allowing 720p portrait encoding.

R=wzh@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/39899004

Cr-Commit-Position: refs/heads/master@{#8303}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8303 4adac7df-926f-26a2-2b94-8c16560cd09d
12 files changed